This application relates generally to disc drives and more particularly to a method and apparatus for sensing the position of a magnetically driven microactuator in a disc drive.
Disc drives are data storage devices that store digital data in magnetic form on a rotating storage medium, such as a disc. Modern disc drives comprise one or more discs that are coated with a magnetizable medium and mounted on the hub of a drive motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks typically by an array of transducers (xe2x80x9cheadsxe2x80x9d) mounted to a radial actuator for movement of the heads relative to the discs. The read/write transducer, e.g. a magnetoresistive read/write head, is used to transfer data between a desired track and an external environment. During a write operation, data is written onto the disc track and during a read operation the head senses the data previously written on the disc track and transfers the information to the external environment.
Typically, such actuators employ a voice coil motor to position the heads with respect to the disc surfaces. The actuator voice coil motor includes a coil mounted on the side of the actuator arm opposite the head arms so as to be immersed in the magnetic field of a magnetic circuit comprising one or more permanent magnets and magnetically permeable pole pieces. When controlled direct current (DC) is passed through the coil, an electromagnetic field is set up which interacts with the magnetic field of the magnetic circuit to cause the coil to move in accordance with the well-known Lorentz relationship. As the coil moves, the actuator body pivots about the pivot shaft and the heads move across the disc surfaces.
The density of concentric data tracks on magnetic discs continues to increase (that is, the radial spacing between data tracks is decreasing), requiring more precise radial positioning of the head. Conventionally, head positioning was accomplished by operating the actuator arm with a large-scale voice coil motor to radially position a head on a flexure at the end of the actuator arm. The head position sensing was achieved by providing the hard disc drive with a servo system that read servo information from the discs and continually repositioned the locations of the transducer heads with respect to the data tracks on the discs.
However, this conventional large-scale actuator motor lacks sufficient resolution to effectively accommodate high track-density discs. Thus, a high-resolution head positioning mechanism, or microactuator, is necessary to accommodate the more densely spaced tracks. These microactuators or secondary actuators enable increased precision for the position servo systems that are used to stay on data tracks with ever increasing track pitch densities. Various microactuator locations and designs have been considered to achieve high resolution head positioning and may operate by a magnetic force, an electrostatic force, or mechanical piezoelectric effect. The microactuators that use magnetic force consist of essentially miniaturized versions of similar magnetic voice coil motors presently used for the larger primary actuators.
However, neither the larger primary actuator nor the magnetic microactuator possess any means of determining their absolute position with respect to each other. Further, when both the primary actuator and the microactuator use magnetic force motors, both motors create motion by applying force or acceleration to the movable portion of the actuators. The position of either actuator therefore represents a mathematical double integration of the applied acceleration. In this case, very small imbalances and offsets will cause one of the actuators to eventually move to the extreme end of its range of motion. This, in turn, will result in failure of any control system to adequately position the actuators.
It is thus desirable to provide a position sensing system which will determine the position of a magnetic microactuator relative to the position of the primary actuator.
Against this backdrop the present invention has been developed for sensing and measuring the position of a transducer attached to a magnetically driven actuator, such as a microactuator, relative to a primary actuator in a head disc assembly.
The head disc assembly in a disc drive has a base plate and a top cover that encloses a drive motor, a disc supported thereon, and a primary actuator assembly. A microactuator carries a read/write head or transducer at a distal end of the primary actuator assembly. The primary actuator assembly provides course or large scale positioning of the transducer over the disc while the microactuator provides fine or small scale positioning of the transducer. The microactuator utilizes a secondary voice coil motor (xe2x80x9cVCMxe2x80x9d) with a coil to adjust the fine position of the transducer.
The position sensing system in accordance with the present invention measures the position of the secondary VCM coil of the microactuator and thus measures the position of the transducer. The position sensing system has a fixed member attached to the primary actuator and a moveable member attached to the fixed member by an attachment device. The attachment device permits the moveable member to move in relation to the fixed member along a predetermined pathway. The secondary VCM coil of the microactuator is fastened to the moveable member. A high frequency signal is applied to this secondary VCM coil. At least one sense coil is attached to the fixed member and positioned near the secondary VCM coil, such that the sense coil will receive a portion of the high frequency signal transmitted from the VCM coil through mutual magnetic coupling. The magnitude of the signal received by the sense coil is inversely proportional to the distance between the coils.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.